Justin,
What kind of darts?
It happened to me for a long time. And while I don't have any scientific analysis to back up this claim, the problem seemed to go away with some additional supplementation. Apparently my female required an additional source of vitamin A.
Ed has referred to this deficiency on many a thread, so I tried it. It worked for me. I had a female that was laying like crazy, and most eggs were perishing around day 12 (for those eggs that didn't perish right away, that is).
I'm cautious to give any specifics about how much to use, as I have no credentials on the matter, nor do I consider myself an expert. But do some searches on vitamin A and tadpole development, and I'm sure a few helpful threads will pop up.
Good luck,
Greg